Cross Reference to Related Applications
The assignee herein is also the assignee of U.S. Pat. No. 4,864,536 and entitled "Optical Memory Method and System," and U.S. Pat. No. 4,915,982 and entitled "Thin Film Photoluminescent Articles and Method of Making Same." The disclosures of both of those patents are incorporated by reference herein.
The present invention relates generally to mass storage devices for data storage. More particularly, the present invention relates to a method of and apparatus for mass data or information storage utilizing purely photoelectronic processes for writing, reading, and erasing stored data.
Optical storage devices presently known generally permit two to three orders of magnitude more data to be stored per disk than with magnetic methods and apparatus. Because of the potential for much greater storage of data and also because of the enormous projected market for such optical memories, active development of optical storage devices is currently occurring in several different directions. Such activities are directed towards read-only, write-once-read many times (WORM) and erasable optical memory systems. While read-only and WORM optical memories are already available, erasable optical memory systems have encountered much greater developmental difficulties than read-only WORM systems because the qualities of the storage media required present problems of much greater technical complexity.
Read-only optical memory devices for use as computer peripherals, such as CD-ROMs, became commercially available with the advent of the digital audio compact disk. Current disk data storage capacity for such units is 200-600 megabytes. Such disks are factory fabricated using a molding press and metalizing operations and are suitable for low cost distribution of large fixed database information.
WORM devices allow the user to encode his own data on the disk, however only once. Data bits are stored at physical locations by irreversibly "burning" the medium with a laser. Such permanent encoding can be read back indefinitely, thus making WORM technology suitable for archival storage of large quantities of information, including digitized images, where random access to a large database is desirable.
It is the third category of optical disk storage devices, namely erasable storage devices, that is believed to embody the greatest utility for mass storage purposes. Such devices will be competitive with present magnetic tape and disk mass storage, and will have a major impact on computer technology in the years ahead. At present, the three most active approaches now being pursued for erasable optical storage involve magneto-optical material systems, dye polymers, and techniques that produce crystal structure or phase transformation in the storage medium at the spot being written to. All of these approaches require heat which usually changes the physical or chemical structure of the materials in performing the write or erase function. Thus, the time to write data to such systems is dependent upon a certain "dwell" time during which the spot to which data is being written must be heated or otherwise physically transformed.
Another drawback with such approaches is that media performance is highly sensitive to impurities, impurity diffusion, oxidation, and other imperfections that propagate into defects and that only show up after multiple switching cycles or at times later than the manufacturing and testing of the devices. Of the three approaches discussed above, progress has been greatest with magneto-optic materials. Laboratory results in this area have reported millions of write/erase cycles. See, for example, H-P. D. Shieh Ph.D. Thesis, Carnegie-Mellon University, Pittsburgh, Pa. (1987).
In order to utilize erasable optical media for mass storage, the optical media must be fast enough to be marked at high data rates using low power lasers. The media must also maintain almost error-free data at acceptable computer industry standards for at least ten years, for example, no more than one uncorrectable error in 10.sup.12 bits. Thus, finding the right physical phenomenon to serve as the basis for erasablity in a high-speed, high-resolution optical storage medium for use with an optical disk storage system or other optical storage system has been very difficult. Most of the effort in the optical disk area over the past ten years, as described above, has gone into the use of magneto-optic materials. However, the commercial realization of erasable magneto-optical storage has not yet been achieved, nor are there yet any guarantees that it ever will be. Unfortunately, the performance of the other approaches discussed above generally is not comparable.
In order to overcome the problems of the prior art, and provide a basis for a workable optical disk storage system, a new approach to the optical storage materials problem which satisfies the optical media requirements of density, speed and long cycle life has been developed. This development utilizes the phenomenon of electron trapping in a class of new materials which comprise an alkaline earth crystal typically doped with rare earth elements. Thin crystalline films of such materials are formed on various substrates, such as glass, polished sapphire or alumina, or other optical quality substrates, in order to provide the disk storage medium.
Since the trapping phenomenon is a purely electronic process, read/write/erase operations can be performed very fast. In addition, the physical trapping phenomenon suggests that media life may be practically limitless. Also, the effect of electron trapping yields a linear response characteristic, which provides an analog dimension to the storage capability. Thus, for example, the potential disk storage capacity of a single 51/4 inch disk could be extended to several gigabytes. Obviously, the density of stored information is extremely high.
The materials to be used as the media for the optical disk storage system described herein are the subject of U.S. Pat. No. 4,915,982, which is a continuation-in-part of U.S. Pat. Nos. 4,864,536 and 4,830,875. Other materials useful as the storage media herein are disclosed in co-pending U.S. Pat. Nos. 4,839,092 and 4,806,772; 4,879,186 and 4,842,960. The assignee herein is the assignee in each of those applications. The disclosure of each of those applications is incorporated by reference herein.
The material described, for example, in U.S. Pat. No. 4,915,982, demonstrates an extremely linear relationship between the intensity of the write input light and the read output light resulting from a fixed-intensity read command. Thus, this capability demonstrates a large noise margin for binary storage, as well as an increased information storage density when employed as an analog or multilevel digital memory medium. Multilevel refers to the fact that by writing with a plurality of intensities of the same laser beam, the linearity of the resulting emissions upon being impinged by a read laser beam is such that information can be stored and recognized at various "levels" of intensity, for example, at 0.2, 0.4, 0.6, 0.8 and 1 intensity.
This particular media is in the form of a thin film and can be "charged" and "discharged" with light by exciting ground state electrons to an elevated energy level. Specifically, upon illumination by visible light, electrons are raised to high energy trapping states, where they can remain indefinitely. When later illuminated by infrared light, the electrons are released from the traps, emitting a new visible light. Thus, with such materials, digital or analog data is stored and retrieved by using low energy lasers to trap and read the electrons at a particular location.
Such solid state photonic materials have electrons having bistable equilibrium states; one with electrons in a ground state, and the other in which electrons are "trapped" in a well-defined, specific, elevated energy state. Electrons are raised to the higher energy state by the absorption of visible light photons, thus filling available trap sites. An electron in the elevated energy state can be released from its trap site by inputting sufficient energy to the electron to permit it to escape from the well. When that occurs, the electron falls back to its ground state and emits a corresponding visible photon. The number of electrons in the elevated energy state is proportional to the visible light intensity used for recording. Thus, as a result of such characteristics, such materials can, in effect, "store" light energy.
The purely photo-electronic mechanisms involved in such electron trapping materials obviate the need for any thermal excursions and, therefore, the number of electrons trapped in the material is inherently linear. Since localized resolution of the "write" step depends only on the performance of the addressing optics, an optical writing spot diameter of one micron will allow at least 550 megabytes of storage on a 130 mm or 51/4" disk coated with a single thin film material as disclosed herein. Multiple layers of thin film materials provide for a like multiple of data storage. For example, two layers of thin film materials will at least double the data storage capacity to 1.1 gigabytes. With the use of encoding techniques such as MFM, modified MFM, or record length limiting (RLL), which techniques are commonly used with magnetic disk recording, the storage capacity can be increased by up to a factor of 3 over the use of FM or frequency modulation coding. The rise and fall times associated with optical read and write pulses are in the nanosecond range. Thus, the read and write data transfer rates have been found to be at least 200 megabits per second for optical disk drive media utilizing electron trapping materials.
Rotating disk memory systems require directions for the retrieval of the stored information. One set of those directions informs the drive mechanism where the requested information is or will be stored. The other set provides alignment for the read/write mechanism during processing. The alignment parameters include focusing, speed, tracks, and mark locations. The writing of information is dependent upon the media used such as write once, magneto-optic, dye polymer, or phase change, but in all cases, involves a change in the reflection parameters in the spot written to. The read method is based on detecting such reflectivity changes at the surface of the disk.
The common method presently used for tracking with reflective surface optical disks, such as the compact disc, is to rely on a grouped track as the principal mode of aligning and focusing the read/write head in the middle of the track. The speed information is either contained in the repetitive pattern of marks or in a depth modulation of the group.
Yet another tracking method presently utilized is known as the "Sampled Servo" system. That system relies on changes in the reflection of spots on the disk surface. The spots are located in a manner which provides information about the track location, the speed of the disk and the adequacy of focusing.
The erasable optical disk memory systems disclosed herein, which rely upon a thin film of electron trapping material as the media, do not rely on reflection for readout. Rather, the emission of the media under infrared stimulation can be utilized to retrieve pre-written tracking information from the disk.
However, even with the advances made by the assignee herein set forth above, the two-dimensional memory system disclosed in U.S. Pat. No. 5,007,037 has certain limitations. With the advent and continued development of parallel processing computers, very fast response memory systems having extremely high density storage capabilities are needed.
While there is much interest and development in two-dimensional erasable optical memories, such an approach will eventually run into an optical resolution limit. That is, a focused beam of light, even a laser, can only be made so small, somewhat less than one micron in diameter. Due to that limitation, only a limited number of bits stored per unit area can be achieved. In order to overcome that physical shortcoming, the present invention utilizes a three-dimensional optical memory storage system, that is, a plurality of at least two layers of different electron trapping materials, each of which responds to light of different wavelengths, in order to greatly increase the storage capacity of, for example, an optical disk memory system equipped with a disk prepared in such a manner.
As an alternative to utilizing "stacked" layers of different characteristic electron trapping materials, a buffered stack of two-dimensional storage planes could also be utilized. Electron trapping material characteristics can be controlled separately, together, or in a defined sequence. Both electron trapping media layers and optical layers can be utilized.